Forms does not send any email or place an entry under Submissions, but shows the success message in the frontend

Hello Balbooa team!

A website of mine does very strange things with Balbooa Forms:
1. Mail in the frontent can be sent
2. ReCaptcha seems to work too
3. Success message also appears.
Now to the problem:
No database entry is set for submissions and emails are not sent.
Unfortunately, we can't find any error messages in any server logs.

Tested without RECaptcha: Works smoothly.

Here is the link to the test page that was set up:

Do you have any idea why this could be?
thanks for your help

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